1. Field of the Invention
This invention concerns an electronic thermal protection device designed to interrupt the power supply on a multiphase line including a temperature detector with a resistive element thermally coupled to a temperature sensor, the latter being connected to a digital processing circuit capable of outputting a control command as a function of the overload detected by the temperature sensor.
2. Description of the Prior Art
Thermal relays are used to protect electrical machines, controlling overloads applied on phases or unbalances or missing phases. A typical thermal relay includes a bimetallic strip for each phase, heated by the phase current. When an overload occurs on one phase, the bimetallic strip for the overloaded phase bends and physically activates a trip mechanism that opens the contact that switches the load off power supply. A differential device is used to activate tripping in the case of an unbalanced phase or a missing phase.
These thermal relays are difficult to use for high currents because the heat input into each bimetallic strip, and therefore the trip current, are not closely controlled since parasite heating occurs.
Electronic relays, such as those described in French patent 2.586.489, are also used to protect machines. These relays include a current sensor producing an image of the phase current, for each phase. An electronic processing circuit receives and processes the phase current images in order to output a trip command.
French patent 2.641.410 describes an electronic relay equipped with a temperature detector comprising a resistive heating element for each phase, through which the phase current passes, and which is thermally coupled to a temperature sensor through a thermal coupling body. This thermal detector is a simple way of storing the thermal condition of the load protected by the relay. This relay uses a resistive element for each phase.
One of the main purposes of the invention is to provide an electronic temperature protection device for a multiphase current using a single temperature detector for all phases, equipped with a single resistance. The temperature detector works at a relatively constant power that reduces temperature increases, makes a more economic design and only requires one resistance when hot. The resistive component of the detector may be made by silk screen printing.